Serial Communications Protocol Specifications - Swissvacuum.com

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03 Illegal data value
04 Failure in associated device
05 Acknowledge: the slave is processing the request
06 Busy: busy device, rejected message
NAK: It is impossible to execute the requested function
07 Memory parity error
For example, let us say that we want to read data from the slave. Master prepares a data packet (query) with modbus
function code 3 (function code 3 identifies a READ request as we will see later) and send the packet to the slave.
In preparing the answer, the slave will return the same function code if no exception is present, otherwise it will return
the exception code as reported below (note that only the first two bytes of the data packet are showed here: the
detailed explanation of the entire frame will follow in the specific paragraph)
0 ADDRESS Chamber controller address
1 03 dec: 00000011 binary Modbus function code for read operation
2 ….
3 ….
4 ….
0 ADDRESS Chamber controller address
1 03 hex: 00000011 binary Modbus function code for read operation
2 ….
3 ….
4 ….
0 ADDRESS Chamber controller address
1 83 hex: 10000011 binary Modbus function code + exception
2 02 Exception number: illegal data address
3 Error check
4 Error check
IDENTIFICATION OF A DATA PACKET
Two different ways are available to the programmer to identify a data packet in binary mode:
� FRAME SYNCHRONIZATION
� LENGHT SYNCHRONIZATION.
In the following section a short description of both will be done in order to have an overview on advantages and
difficulties of both.
FRAME SYNCHRONIZATION
QUERY FROM THE MASTER TO THE SLAVE
ANSWER FROM THE SLAVE, NO EXCEPTION
ANSWER FROM THE SLAVE, EXCEPTION 02
Frame synchronization can be manteined in binary transmission mode (RTU) only by simulating a
synchronous message. The receiving device monitors the elapsed time between receipt of characters.
Modbus specifications requests that this time (time between the reception of a character and the following) is
less or equal to 3.5 times the time of character transmission because the received data can be considered
Serial communications protocol vers. 2.02 Angelantoni Industrie Spa Massa Martana (PG) Italy